1. Field of the Invention
The present invention relates to a method for pneumatic length measurement in which the medium used for the measurement is directed through a pre-nozzle and a measuring nozzle onto an impact plate and the spacing of the impact plate from the measurement nozzle is determined from a change of the pressure p2 between the pre-nozzle and the measurement nozzle.
2. Description of the Prior Art
Methods for pneumatic length measurement have been known for a long time and are in particular used in the manufacture of workpieces because of the high measurement accuracy, the rapid determination of the measured value and the insensitivity of the measurement method with respect to external influences.
A disadvantage of this method lies in the fact that the supply pressure of the measurement apparatus must be constant in order to be able to guarantee an adequate measurement accuracy. So-called bridge circuits are thus also known, in which the pressure is measured between a measurement branch and a reference branch. Because air pressure fluctuations of the supply pressure act equally on the reference branch and on the measurement branch, these fluctuations are compensated. The disadvantage of the bridge circuit, however, lies in the fact that twice the quantity of air is consumed through the reference branch. Moreover, the manufacturing costs and the complexity of the adjustment are increased by the reference branch.
The invention is based on the object of setting forth a pneumatic method of length measurement of the initially named kind, which has a high measurement accuracy despite fluctuations of the supply pressure and which, at the same time, enables a low consumption of compressed air and low manufacturing costs.
This object is solved in that the pressure p1 in front of the pre-nozzle is additionally measured and the length value resulting from the pressure measurement between the pre-nozzle and the measurement nozzle is corrected in dependence on the pressure p1.
In accordance with the invention a correction of the pressure measured between the pre-nozzle and the measurement nozzle of the measurement branch, i.e. of the length value derived therefrom, is effected instead of a bridge circuit. The correction takes place in this respect in dependence on the additionally measured pressure p1 before the pre-nozzle. This additionally measured pressure p1 is a measure of the fluctuations of the air supply pressure and can thus be used for the correction. However, the correct length value cannot be precisely calculated from the additionally measured pressure p1. An approximation function is, however, sufficient as a correction in order to increase the measurement accuracy in the desired manner.
In accordance with one embodiment of the invention, the following function is used to calculate the length instead of the pressure p2:
f(p1, p2)=xcex1p1-p2
wherein a is a basically freely selectable constant. Through this approximation function a considerable improvement of the measured values is already achieved. It is preferred if a suitable value for a is determined by measurement of the pressures p1 and p2 at, in each case, two different air supply pressures A and B and by calculation of ac in accordance with the formula:   α  =                    P                  2          ⁢          B                    -              P                  2          ⁢          A                                    P                  1          ⁢          B                    -              P                  1          ⁢          A                    
wherein P1A . . . p1 at air supply pressure A
P1B . . . p1 at air supply pressure B
P2A . . . p2 at air supply pressure A
P2B . . . p2 at air supply pressure B
is determined. It has been shown that a high measurement accuracy is achievable herewith.
In accordance with a further embodiment of the invention, the two air supply pressures A and B are selected symmetrically to a desired air pressure, in particular to the intended air pressure, for the determination of xcex1. The measurement is executed at the desired spacing X0 of the measurement nozzle. Through this design it is ensured that the highest measurement accuracy is achieved in the region of the intended air pressure and of the desired distance. A change of the desired distance can in this respect be realized simply by a change of xcex1. That is to say, a zero point shift for the adjustment of the measuring apparatus can be effected via a: without mechanical changes at the measurement apparatus.
In accordance with a further embodiment of the invention, the correction is carried out in accordance with the formula:
xe=xxe2x88x92xcex3xcex94pxcex94x
where xcex94p=p1xe2x88x92P0 and xcex94x=xxe2x88x92X0, with
xe . . . being the corrected length value
x . . . being the length value calculated from p2 
xcex3 . . . being a basically arbitrarily selectable constant.
Through this embodiment an even higher accuracy of the measured values can be achieved, in particular with larger air pressure fluctuations and larger deviations of the distance from the desired value.
In accordance with a further embodiment of the invention, a suitable value for xcex3 is determined by determination of the indicated length value at, in each case, two different distances set by differing measurement and at two different pressures p1 and by calculation of xcex3 in accordance with the formula:   γ  =                    X                  2          ⁢          Ph                    -              X                  2          ⁢          P0                                    (                              P            h                    -                      P            0                          )            ⁢              (                              X                          2              ⁢              Ph                                -                      X                          0              ⁢              P0                                      )            
with
X2Ph . . . being the indicated length value at a distance X2 and pressure Ph 
X2P0 . . . being the indicated length value at a distance X2 and pressure P0 
X0P0 . . . being the indicated length value at a distance X0 and pressure P0.
It has been shown that particularly precise measurement values can be achieved with this xcex3 value.
In accordance with a further design of the invention, X0 is selected as the desired spacing, and P0 as the desired pressure. Through this embodiment the highest accuracy is advantageously achieved at the desired spacing and at the desired pressure.
In accordance with a further embodiment of the invention, p1 and p2 can be calibrated values of the pressure sensors before the pre-nozzle and between the pre-nozzle and the measurement nozzle. In accordance with another design, p1 and p2 can be uncalibrated values of these pressure sensors, and in particular their voltage values v and u.
When using uncalibrated pressure values the calculation of the corrected length value takes place in accordance with a further embodiment of the invention, preferably in accordance with the formula:             x      e        =                  X        1            +                        (                                                    f                e                            ⁢                              (                                  v                  ,                  u                                )                                      -                          F              1                                )                ⁢                                            X              2                        -                          X              1                                                          F              2                        -                          F              1                                            ,
wherein
fe(v, u)=f(v, u)xe2x88x92xcex3xcex94vxcex94f(v, u)
xcex94v=vxe2x88x92V0 
xcex94f(v, u)=vxcex1xe2x88x92uxe2x88x92F0 
F1=fe(v, u) at a distance x=X1 and
F2=fe(v, u) at a distance x=X2 
with
xe . . . being the corrected length value
v . . . being the uncalibrated value of the pressure sensor before the pre-nozzle
u . . . being the uncalibrated value of the pressure sensor between the pre-nozzle and the measurement nozzle.
With this formula a very precise measurement value can be calculated without the calibrated pressure sensors being necessary. Accordingly, cost saving, uncalibrated pressure sensors can be used for the measurement apparatus.